Shielding a magnetic core



Aug. 22, 1967 2 H 1 I 7 "w A 2 .4 M v 3 4 W V G B /1l B H F O l l w 7 e i l 1 7234 .l 1 2 drllmr nrlfr' J I H I a Q I "Q I. I1 2 m W F El 7 6 INVENTORS. BRADFORD HOWLAND BY ROBERT s. BERG 441/ Q. flaw AGENT FIG.

United States Patent 3,336,662 SI-IllELDING A MAGNETIC CORE Bradford Howland, Cambridge, and Robert S. Berg, Lexington, Mass, assignors to Massachusetts Institute of Technology, Cambridge, Mass., a corporation of Massachusetts Filed June 7, 1962, Ser. No. 200,748 4 Claims. (Cl. 29-602) This invention relates to a low leakage-inductance transformer and in particular to a high frequency transformer with a magnetic core shielded from the windings by a chemically and electrically deposited electrostatic shield.

A transformer capable of operating over a frequency band of approximately 1 to 100 megacycles finds many applications in electrical circuits which cover this frequency range. Satisfactory frequency response has been obtained by transformers constructed in accordance with the invention of this application in hybrid junction or magic T circuits where a turns ratio approximating \/2:1 is required. Distributed transformer techniques, while providing the necessary bandwidth were not used because of the difiiculty in getting the required turns ratio. The use of the eddy-current shield of the present invention reduces the leakage inductance by an order of magnitude with consequent improvement in the high frequency response of a transformer wound by conventional techniques.

Prior art transformers have used eddy current shielding to reduce leakage inductance. One type of eddy current shielding which is used on a toroidal core consists of a copper screen having a circumferential gap along the inner surface of the toroid. Such a shield is discussed in an article in the Wireless Engineer, June 1947, pp. 175- 176 where it is stated that the windings of the transformer may be placed outside the screen. The general outline of such a shield is similar to that shown in FIGURE 3 of this application. Another type of shield which is described in the General Radio Experimenter, vol. XXX, No. 11, April 1956, consists of two copper toroidal cups of different diameters which slip over a toroidal magnetic core to form a shield similar to that of FIGURE 5 of this application. Neither of these techniques are suitable for application to magnetic cores especially suited for high frequency transformers. These cores are physically of small dimensions since transformers in general improve in high frequency performance as the size decreases. Since space for the transformer windings is at a premium, further reduction of this winding space by eddy current shielding must be kept to a minimum. The copper screen shield is bulky and not suitable for small cores. The copper cups are difficult to fabricate in small sizes and with the thin walls required to avoid occupying a large portion of the center hole winding space of the toroidal core. In addition, copper cups are restricted to use with toroidal cores since fabrication of the cups for other shapes by machining techniques would be difiicult.

Accordingly, it is an object of this invention to provide a means for eddy current shielding of small magnetic cores which is easy to fabricate and which reduces the available winding space by a negligible degree.

It is a further object of this invention to provide an eddy current shield which may be easily fabricated on cores of shape other than toroidal.

These objects are obtained in the present invention by chemical and electrical deposition of a conducting surface over the entire core and by removing said conductor along a circumferential path to avoid a short circuited turn. A second conducting surface applied on a nonconductive coating on said first conducting surface and similarly modified to avoid a short circuited turn will provide additional reduction in leakage inductance.

The novel features of the invention together with further objects and advantages thereof will become apparent from the following description taken in connection with the accompanying drawings wherein:

FIGURE 1 shows in cross section a single layer conductive material eddy current shield on a toroidal core. FIGURE 2 shows in cross section a two layer shield.

FIGURE 3 shows in cross section another single layer shield with a gap on a circumference of the core.

FIGURE 4 shows in cross section the preferred embodiment of the two layer shield.

FIGURE 5 shows in cross section a multiaperture core with windings on the legs thereof.

FIGURES lthrough 4 show in cross section a toroidal core 1 on which a copper plated shield 2 has been deposited. Copper shield 2 serves as the eddy current shield which is of a shape inconvenient for direct mechanical fabrication and is most easily formed by electroplating directly on the core. The purpose of shield 2 is to provide an eddy current barrier to a changing magnetic flux with a component normal to shield 2. This barrier prevents fiux from leaving core 1 and looping around either coil 6 or 7 before reentering the core 1, thereby producing a leakage inductance in whichever coil is looped. It is apparent that shield 2 cannot be perfectly effective since it must be interrupted along some circumference to avoid a shorted turn. One way of effecting this interruption is shown in FIGURE 1 where a countersink has been used to remove the plating at the inside corner 5 of the core 1. Care must be exercised in controlling the amount of material removed from corner 5 lest too little material removal causes an inadvertent short circuit, while too much removal will cause an excessive amount of flux to escape from the core 1 through gap 9, thereby increasing the leakage inductance of either or both coils 6 and 7. Another difiiculty with the removal of material from a corner of the core is that the core must be sufficiently uniform in inside diameter and thickness that material is removed uniformly from the corner 5 of the core 1.

FIGURE 2 is a cross section of a toroidal magnetic core 1 having a two layer shield consisting of shields 2 and 4 separated by a nonconductive material 3. The short circuited turn produced by shield 4 is interrupted by using a countersink to remove enough of the shield at corner 8 to form gap 10 in the same manner as the gap 9 at corner 5. In FIGURE 2, corners 5 and 8 are chosen as the inside corners of core 1. To reduce leakage flux to a minimum, the gaps 9 and 10 should be at opposite corners although for convenience of fabrication, both inside or both outside corners may be used.

FIGURE 3 shows a core 1 in which the copper shield 2 is interrupted by a circumferential cut 11 made by a fine slitting saw. The difficulty of controlling the depth of the saw cut 11 makes this embodiment of the invention somewhat less desirable than FIGURE 1 for a two layer shield configuration. The width of the saw cut 11 should narrow to minimize flux leakage but for small sized cores, available saws make a wider cut than is desirable.

FIGURES 1 and 3 are about equally effective, giving a four fold reduction of leakage inductance for the case of oppositely disposed primary 6 and second 7 windings on a high permeability toroidal core 1 as compared to the case where no shield 2 is used.

FIGURE 4 shows in cross section the preferred embodiment of this invention. A toroidal core 1 with interlocking separated shields 2 and 4 is shown. The magnetic reluctance of the flux leakage path in FIGURE 4 is considerably greater than in FIGURES 1 and 3. An eightfold reduction of leakage inductance has been obtained with the construction of FIGURE 4 as compared to a core with no shielding. The construction of FIGURE 4 has the further advantage that no precision machining operations are required,.and there is little possibility of a short circuit causing a shorted turn.

Magnetic core 1 is a ferrite core suitable for operation at high frequencies. Typically, successful transformers have been constructed using ferrites manufactured by the General Ceramics Company in toroidal core size CF-l02 (approximately OD. x /6" ID. x /s), tumbled to remove sharp edges. The ferrites Q-l and Q-Z have high bulk resistivity and may be copper plated directly; type H is of lower resisitivity, and for best results is coated with a thin film (3-5 mils.) of a nonconductive plastic of good dielectric properties before copper plating. An acrylic type plastic coating has been found to work satisfactorily. The plastic may be applied by any process suited to the particular plastic used which will produce a thin, relatively uniform coating free of pin holes and preferably free of high spots. Spraying of this plastic has been found to produce satisfactory results.

The copper plating shields 2 and 4 are applied by a chemical deposition technique followed by conventional electroplating to the desired thickness, 4-5 mils in the case of the shield 2 of FIGURES 1 and 3 and 2-3 mils for the shields 2 and 4 of FIGURE 4. A suitable deposition technique is described in the book Metallizing of Plastics by Narcus, pp. 14-39, Reinhold Publishing Corp, 1960.

In the construction of the embodiment of FIGURE 4, the first copper plating shield 2 is applied over a plastic coating (not shown) if required by the core 1 resistivity, otherwise directly on the core. The shield 2 is removed from surface 12 of core 1 by rubing surface 12 on fine emery paper, being careful to remove sufficient copper plating to avoid a short circuited turn. Next, a layer of 5-7 mils of plastic coating 3 is applied as described previously. This is followed by a second copper plating 4 of 2-3 mils thickness over the entire plastic coating 3. The plating is removed from surface 13, which is opposite to surface 12, by rubing surface 13 on fine emergy paper. Care should be exercised in this last operation to remove sufficient plating to avoid a short circuited turn while not removing so much material that plating shield 2 is removed also. Windings 6 and 7' may then be wound on the completed core assembly to produce a transformer.

Although the emobidment shown in FIGURE 4 shows a core 1 of a toroidal for-m, the same technique for shielding has been sucessfully applied to transformers where a three-legged core has been used, shown in cross section in FIGURE 5. The third leg 14 of the core 1 is in the same plane as the legs 15, 16 of core 1 and of the same thickness. Thus, the copper plating may be removed from the faces 12 and 13 in the same manner as described for the toroidal core of FIGURE 4. The Windings 6, 7, and. 17, insulated from plating 4, are wound on legs 14, 15 and 16 to produce a transformer having low leakage inductance and good high frequency performance. The core of FIG- URE 5 is approximately twice the size of the toroidal cores of FIGURES 1 through 4, but with about the same thickness.

Transformer construction with the cores of FIGURES 1 through 5 consists of winding the core 1 with Teflon or plastic insulated wires 6, 7 (and 17 in FIGURE 5) of thickness chosen to give the correct winding impedance. For most applications of these transformers, the outer plating shield 4, if a two layer shield is used, will be grounded to minimize electrostatic coupling between the windings 6, 7 (and 17 in FIGURE 5). The inner shield 2 of FIGURES 2, 4 and 5 being imperfectly exposed to the windings and having large capacitance to the outer shield 4, need not be grounded.

In orderto avoid severe loss of Q with certain ferrites, such as the types Q-l and Q-2 ferrites, following the plating and interupting operations, it was found desirable to dry the cores for a day at room temperature in a vacuum dessicator after each plating and interrupting operation. It is believed that the Q of the ferrite cores was reduced because of moisture absorption which caused a large decrease in the high frequency bulk resistivity of these porous ferrite materials.

Transformers for wide bandwidth and high frequencies generally require very few turns and may be handwound to a variety of specifications without difficulty. Several single-layer windings spaced above a grounded, shielded core will exhibit effective electrostatic isolation together with close magnetic coupling. It is seen that the construction of this'invention relegates most of the difficulties to the preparation of the cores, of which only a few types and sizes would normally be required.

Table I includes the performance specifications of a transformer intended for use in a balanced mixer circuit.

TABLE I Core CFl05-H (General Ceramics) toroid.

Shield Double plated as in FIG- URE 4.

Fri. impedance ohms.

Sec. impedance 360 ohms, CT.

Pri. winding 8 turns No. 32 TENSILITE wire.

Sec.winding 16 turns CT, same No. 32

wire.

Primary inductance 36 'microhenries.

Leakage inductance (referred to primary) Sec. unbalanced cap less than 0.1 mmf. Freq. response (3 db down) 0.3 to me.

0.22 microhenry.

We claim:

1. A method for fabricating a low flux leakage magnetic circuit comprising chemically depositing an electrically conductive non-magnetic material as a first coating to completely encompass a toroidal core of magnetic material having two diametric surfaces substantially perpendicular to the axis of the toroid, removing the conductive coating from one of the two surfaces, depositing an electrically insulating material as a second coating on the conductive coating and said surface, chemically depositing an electrically conductive non-magnetic material as a third coating over the insulating coating, removing the conductive coating from the other of the two surfaces whereby the two conductive coatings provide an eddy current shield.

2. A method as described in claim 1 comprising in addition abrasively smoothing the two surfaces of the toroid to obtain essentially planar surfaces before depositing the first coating.

3. A method of producing a low leakage flux path mag netic core comprising chemically depositing an electrically conductive non-magnetic film to completely encompass said core, removing a portion of said film to form a first continuous gap parallel to the flux path in said core, coating said non-magnetic conductive film and said gap with a layer of electrically insulating, plastic, chemically depositing a second electric-ally conductive non-magnetic film on said plastic coating, removing a portion of said second film to form a second continuous gap parallel to said flux path in said core, said second gap being formed on the opposite side of said core from said first gap.

4. A method for fabricating a low flux leakage magnetic circuit comprising chemically depositing an electrically conductive coating on the entire surface of a multi-apertured core, said core having two or more continuous flux paths within said core, said core having two planar diametric surfaces each of which contact all flux paths, removing the conductive coating from one of the two planar 5 6 surfaces, depositing a coating of insulating material over 3,063,135 11/1962 Clark 29155.5 the entire surface of said core including said one plane 3,123,787 3/1964 Shifrin 29-15556 X surface, chemically depositing a second electrically con- 3,149,296 9/1964 Cox 336-84 ductive coating over said insulating material, removing 3,154,840 11/1964 Shahbender 340174 isgssficond coating from the other of the two plane sur- 5 FOREIGN PATENTS References Cited 229,109 11/1909 Germany.

UNITED STATES PATENTS JOHN F. CAMPBELL, Primary Examiner. 2,939,096 5/1960 Gordon 33684 L A E 3,010,185 11/1961 Hume 29-155.5 10 E 3 032 729 5 19 2 pluegel 33 g4 M. W. COOK, R. W. CHURCH, Assistant Examiners. 

3. A METHOD OF PRODUCING A LOW LEAKAGE FLUX PATH MAGNETIC CORE COMPRISING CHEMICALLY DEPOSITING AN ELECTRICALLY CONDUCTIVE NON-MAGNETIC FILM TO COMPLETELY ENCOMPASS SAID CORE, REMOVING A PORTION OF SAID FILM TO FORM A FIRST CONTINUOUS GAP PARALLEL TO THE FLUX PATH IN SAID CORE, COATING SAID NON-MAGNETIC CONDUCTIVE FILM AND SAID GAP WITH A LAYER OF ELECTRICALLY INSULATING, PLASTIC, CHEMICALLY DE- 